The present invention pertains to a sputtering target for a phase change memory capable of reducing, as much as possible, impurity elements that affect the crystallization speed, reducing the compositional deviation of the target in relation to the intended composition, and improving the rewriting characteristics and crystallization speed of the phase change memory by suppressing the compositional segregation of the target, as well as to a phase change memory film formed with such a target and the manufacturing method of this target.
In recent years, high-density memory optical disc technology capable of storage/reproduction without requiring a magnetic head has been developed, and is rapidly attracting attention. This optical disc can be classified into the three categories of reproduction-only, recordable and rewritable. Particularly, the phase change method employed in the recordable and rewritable type discs is attracting attention.
This phase change optical disc performs the storage/reproduction of information by heating and increasing the temperature of a memory thin film on a substrate by irradiating a laser beam thereto, and generating a crystallographic phase change (amorphouscrystal) in the structure of such memory thin film. More specifically, the reproduction of information is conducted by detecting the change in the reflectivity caused by the change in the optical constant of the phase.
The aforementioned phase change is performed with the irradiation of a laser beam narrowed down to a diameter of approximately 1 to several μm. Here, for example, when a 1 μm laser beam passes through at a linear velocity of 10 m/s, light is irradiated to a certain point on the optical disc for 100 ns, and it is necessary to perform the aforementioned phase change and detect the reflectivity within such time frame, and realize the foregoing crystallographic phase change; that is, the phase change of the amorphous and crystal.
In light of the above, a phase change optical disc has a four-layer structure wherein, for instance, both sides of a Ge—Sb—Te or In—Sb—Te memory thin film layer or the like are sandwiched with a zinc sulfide-silicic oxide (ZnS—SiO2) high-melting point dielectric or the like, and an aluminum alloy reflective layer is additionally provided thereto. Stibium (Sb), tellurium (Te) or selenium (Se) is an important constituent element as the foregoing optical memory medium.
Moreover, recently, a nonvolatile phase change memory has been proposed which forms a chalcogenide film with sputtering, makes an electrode contact such film, and generates a phase change of the chalcogenide by flowing current to this electrode, and this technology is attracting attention. The nonvolatile memory employing this type of technique is generally referred to as a PRAM or OUM (Ovonic Unified Memory).
To explain the outline of this OUM, when a chalcogenide sputtering thin film is partially heated to 600° C. or higher, an amorphous phase is formed upon quick cooling of 1 to 2 ns. Here, the heated area is narrow, and, although the center will reach 600° C. with a device measurement of 50×200 nm2, there is data stating that at 100 nm away, the temperature only rises to 100° C.
Although crystallization will not occur with the quick cooling described above, crystallization will occur when this is annealed for 20 ns to 50 ns at 300 to 400° C. Crystallized chalcogenide has low resistance, but amorphous has high resistance. And, in either state, the characteristics will be inverted when exceeding the threshold voltage.
OUM utilizes the foregoing characteristics, and has numerous advantages such as nonvolatility, high density, low voltage, low power consumption, rewriting is possible 1012 times, nondestructive readout, integration with the Si process is easy, it may be made into a unified memory, and so on.
A phase change optical disc and OUM both employ a chalcogenide sputtering thin film formed from the element of stibium, tellurium or selenium, and sufficient consideration must be given to the characteristics of the material.
Nevertheless, when impurities get mixed into this type of memory medium, such impurities would get condensed near the boundary face of the memory point and non-memory point together with the repetition of the phase change between the liquid phase-solid phase in connection with the deletion of memory, and a crystal growth nucleus that will become the source of coarse crystal grains in the vicinity of the memory point will arise, whereby the number of times rewriting may be conducted is reduced as a result thereof.
The memory thin film layer, which is this type of phase change memory medium, is usually formed with the sputtering method as described above. This sputtering method makes a positive electrode target and a negative electrode target face each other, and generates an electric field by applying a high voltage between the substrates thereof and the targets under an inert gas atmosphere. The sputtering method employs a fundamental principle where plasma is formed pursuant to the collision of electrons, which are ionized at such time, and the inert gas, the positive ion in this plasma extrudes the atoms structuring the target by colliding with the target (negative electrode) surface, and the extruded atoms adhere to the opposing substrate surface, wherein the film is formed thereby.
The target used for sputtering in itself contains numerous impurities, and, when the deviation in relation to the intended composition is significant, this will considerably affect the memory thin film layer, and there is a problem in that the number of times rewriting can be conducted will decrease as a result thereof.
In light of the above, several high purity and high density targets have been proposed. This type of manufacturing method conventionally proposed is a manufacturing method combining the dissolution method and powder metallurgical production. Nevertheless, the elements of stibium, tellurium and selenium have high vapor pressure, and, since these elements preferentially evaporate during dissolution, they tend to deviate from the intended composition, and there were additional drawbacks in that segregation of the composition would occur in the target. Compositional deviation in the recording film causes the unevenness in the crystallization speed, and also adversely affects the rewriting characteristics.
In consideration of the above, the compositional deviation was either neglected, or the composition was prepared in anticipation of such compositional deviation or compositional segregation. With respect to the latter, although there are cases that the intended composition is fortunately obtained, the precision of compositional evenness and reproducibility of deposition are inferior, and there is a drawback in that the product will be unstable.
Further, since the foregoing methods connive at the evaporation in the heating furnace, there is a problem in that the inside of the furnace gets contaminated for each process, and, while dissolution is being repeated, contamination arises from gas components, inside of the furnace and from crucible materials, and there is a problem in that it is difficult to maintain the target at a high purity.